The USGS is developing a 3-D seismic velocity model for the San Francisco Bay Area primarily for the calculation of realistic strong-ground-motion synthetic seismograms for local earthquakes. The model, gridded at 100-m intervals, extends from the ground surface to the Moho, from the coast to the Great Valley, and from the Geysers (38.8°) to Watsonville (36.9° ... ). The model is a hybrid of geological and geophysical components; the Cenozoic sedimentary cover is defined primarily by geologic data and the pre-Cenozoic units primarily by geophysical data. The model boundaries include: (1) elevation and bathymetry, (2) major strike-slip faults, (3) contacts between surficial geologic materials, (4) base of the Holocene Bay Mud, (5) base of Quaternary alluvium, (6) top of Franciscan or Salinian basement inferred from an inversion of gravity data and constrained by outcrop, well control, and seismic profiles, (7) top of lower crust, and (8) the Moho. S- and P-wave velocities for fine, medium, and coarse-grained Quaternary alluvium have been compiled for >120 shallow boreholes in the South Bay. Fifty sonic and density logs from oil test wells have been compiled to determine typical P-wave velocities and densities of the Cenozoic sedimentary units. A P-wave tomographic velocity inversion is used to constrain the velocity model between the top of Franciscan or Salinian basement and the top of the lower crust, at about 16 to 18 km. Velocity models from more than 20 existing deep-crustal seismic refraction lines have been compiled to determine the geometries and velocities of the lower crust and Moho. Important features of the model with implications for strong ground motion include the presence of several deep (>3 km) sedimentary basins which may locally enhance ground motions and the regional presence of a strong S- and P-wave reflector at the top of the lower crust producing prominent peaks in ground motions at ranges between 40 and 60 km.

Compilation of Oil Test Well Sonic and Density Log Data

Several relatively thick (>3 km deep) Cenozoic basins, including the Cupertino, Evergreen, Livermore, and San Pablo basins, may locally enhance strong ground motions in the San Francisco Bay area, California. As part of a crustal-scale, three-dimensional seismic velocity and density model for the Bay area, we have compiled data from sonic and density logs from oil test wells in the Bay area to better understand strong motion resonances generated by these basins. We have compiled the velocities and densities of sediments and rocks within these Cenozoic basins using 59 sonic and density logs from 51 oil test wells. The well data are primarily from the Livermore, Concord, and Los Medanos oil fields, and the Sacramento-San Joaquin delta, and provide measurements from the surface to as much as 5.3 km subsurface. Only a few logs from the South Bay are included in this compilation. The logs were hand digitized at non-uniform intervals between 3 and 30 m to capture the significant variations of the logs with depth for frequencies up to 2 Hz. Linear regression through 41 sonic logs yields Vp (km/s) = 2.24 + 0.599Z, where Z is depth in km. Shallow borehole data, generally from the South Bay, and from less than 30 m deep, indicate that the average surficial P-wave velocity at 10 holes in weathered Tertiary sedimentary units ranges from 2.21 and 2.32 km/s and is in close agreement with extrapolated P-wave velocities inferred from the oil test wells. A sonic log for Eocene sediments from Butano Ridge in San Mateo County shows that at a given depth, velocities are approximately 0.5 km/s higher than those near Livermore. The higher P-wave velocities for the Tertiary sedimentary rocks at Butano Ridge probably result from a combination of dense volcanic clasts in conglomerates plus very tight compaction of the sandstones. Density logs in Cenozoic sedimentary rocks show higher scatter. Linear regression of 18 density logs yield r (g/cm3) = 2.25 + 0.065Z. Average densities of weathered Tertiary sedimentary rocks measured on core samples from 5 shallow boreholes in the South Bay lie between 2.20 and 2.25 g/cm3, in close agreement with the surficial density inferred from linear regression of oil well data. This report presents the locations, elevations, depths, stratigraphic and other information about the oil test wells, provides plots showing the density and sonic velocities as a function of depth for each well log, and compiles all data to better understand the velocities and densities of Cenozoic sedimentary rocks in the Bay area. [Summary provided by the USGS.]

Access Constraints
Well log data are available in excel4 or excel5 or ascii format. The anonymous ftp address is: eratos.wr.usgs.gov. Password in required. Change the directory (cd) to /pub/sfbay/welllogs. The files are named SFBay.sonic.xl4.bin and SFBay.density.xl4.bin (Excel4) and SFBay.sonic.xl5.bin and SFBay.density.xl5.bin (Excel5), in Mac Binary II format. Table 1 of this report in Excel5 format is also in this ftp site, labeled as Table 1.OFR.bin.